A comprehensive understanding of permeability and energetic factors which influence active sodium transport requires knowledge of pertinent flows and forces, as well as concentrations of rate-determining metabolic intermediates. In general, values of the electrical potential difference across permeability barriers and the affinity (negative free energy) of metabolic driving reaction(s) are unknown; microelectrode measurements of intracellular potential are traumatic and necessarily brief; tissue metabolite measurements provide only mean concentrations, rather than local concentrations in the region of the sodium pump. Neither technique is readily applicable to multiple sequential observations. For these reasons, we propose to use rapid non-traumatic optical techniques (scanning differential spectrofluorometry) in the toad bladder. The fluorescence of extrinsic "probes" will indicate intracellular electrical potential profiles. Intrinsic fluorescence will reflect NADH conentrations. The relation of extrinsic and/or intrinsic fluorescence intensity to transmembrane potential in control and experimental states will help to discriminate between effects on conductances at the two rate-limiting permeability barriers and effects on energetic factors. This fluorescence technique is compatible with procedures presently employed to measure rates of active and passive sodium transport and oxygen consumption and to evaluate the affinity. The combination of techniques would allow a comprehensive analysis of permeability and energetics and thus test the adequacy of current models and presumed mechanisms of drugs and hormones modifying transport.